errno 100 (ENETDOWN): meaning and diagnosis

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: the local network interface selected for the socket operation is down or unavailable.

Could be also:
Constant	Type	OS
ERROR_TOO_MANY_SEMAPHORES	Win32 error	Windows
HTTP_STATUS_CONTINUE	HTTP Code	Any
EPROTO	errno	Mac
EADDRINUSE	errno	Windows
SYMBOLIC_INITIALIZATION_FAILED	BugCheck Code	Windows

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ENOPROTOOPT	EPROTO

ENETDOWN

ENETDOWN is a local network-stack failure. It says that the interface needed to reach the destination is down, rather than that the remote service rejected a connection. A send or connect can therefore fail before a packet is delivered to the peer. This is especially relevant when an application has more than one route, runs in a network namespace, or uses a virtual, VPN, tunnel, or container interface.

Keep it separate from ENETUNREACH, which describes the absence of a route to a network, and from EHOSTUNREACH, which is normally more specific to a destination host. Bringing up an unrelated interface or restarting the remote service does not answer which egress interface and route the failing socket actually selected.

Evidence that identifies the failing path

Record the operation, destination address, address family, local address if already bound, and the socket's network namespace.
Run ip route get for the exact destination and inspect the chosen device, source address, route table, and policy rule.
Inspect that device with ip link show; distinguish administrative state, carrier state, and the existence of the device in the namespace.
For a managed service, correlate the failure with interface, DHCP, VPN, container, or network-manager events rather than retrying the same request without a delay policy.

POSIX sendto() · Linux connect(2) · Linux ip-link(8)

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